Glycogen synthase kinase 3 (GSK3) is a serine/threonine protein kinase composed of two isoforms (α and β), which are encoded by distinct genes but are highly homologous within the catalytic domain. GSK3 is highly expressed in the central and peripheral nervous system. GSK3 phosphorylates several substrates including tau, β-catenin, glycogen synthase, pyruvate dehydrogenase and elongation initiation factor 2b (eIF2b). Insulin and growth factors activate protein kinase B, which phosphorylates GSK3 on the serine 9 residue and inactivates it.
GSK3 and Bone Disorders
Remodeling of the skeleton is a continuous process, controlled by systemic hormones such as parathyroid hormone (PTH), local factors (e.g. prostaglandin E2), cytokines and other biologically active substances. Two cell types are of key importance: osteoblasts (responsible for bone formation) and osteoclasts (responsible for bone resorption). Via the RANK, RANK ligand and osteoprotegerin regulatory system these two cell types interact to maintain normal bone turnover (Bell N H, Current Drug Targets—Immune, Endocrine & Metabolic Disorders, 2001, 1:93-102).
Osteoporosis is a skeletal disorder in which low bone mass and deterioration of bone microarchitecture lead to increased bone fragility and fracture risk. To treat osteoporosis, the two main strategies are to either inhibit bone resorption or to stimulate bone formation. The majority of drugs currently on the market for the treatment of osteoporosis act to increase bone mass by inhibiting osteoclastic bone resorption. It is recognized that a drug with the capacity to increase bone formation would be of great value in the treatment of osteoporosis as well as having the potential to enhance fracture healing in patients.
Recent in vitro studies suggest a role of GSK3β in osteoblast differentiation. First, it has been shown that glucocorticoids inhibit cell cycle progression during osteoblast differentiation in culture. The mechanism behind this is activation of GSK3β in osteoblasts, resulting in c-Myc down-regulation and impediment of the G1/S cell cycle transition. The attenuated cell cycle and reduced c-Myc level are returned to normal when GSK3β is inhibited using lithium chloride (Smith et al., J. Biol. Chem., 2002, 277:18191-18197). Secondly, inhibition of GSK3β in the pluripotent mesenchymal cell line C3H10T1/2 leads to a significant increase in endogenous β-catenin signaling activity. This, in turn, induces expression of alkaline phosphatase mRNA and protein, a marker of early osteoblast differentiation (Bain et al., Biochem. Biophys. Res. Commun., 2003, 301:84-91).
Published in vivo data confirming the in vitro effects of GSK3β on osteoblast differentiation are still lacking. However, studies by the inventors clearly show an increased bone formation in rats treated with a GSK3β inhibitor (see below under Examples). It should also be noted that patients treated with lithium have increased levels of bone-specific alkaline phosphatase, indirectly providing support for the notion that inhibition of GSK3β would lead to osteoblast stimulation and increased bone formation (Broulik et al., Clinica Chemica Acta, 1984, 140:151-155).